Fix compile errors on Unity 6000.5 (CS0619) and 6000.6 (UAC1016)

[lordborden]

Two independent compile errors block Scopa on current Unity releases. Both fixes are small and backward-compatible.

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1. Unity 6000.5 — CS0619 in MapImporterEditor

Unity 6000.5 promotes two APIs from obsolete-warning to obsolete-as-error (CS0619):

• SerializedProperty.objectReferenceInstanceIDValue
• Object.GetInstanceID()

Fix: switch to the EntityId replacements Unity's own obsolete messages recommend, guarded so older editors keep the existing path.

#if UNITY_6000_5_OR_NEWER
    externalConfig.objectReferenceEntityIdValue = configAsset.GetEntityId();
#else
    externalConfig.objectReferenceInstanceIDValue = configAsset.GetInstanceID();
#endif

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2. Unity 6000.6 — UAC1016 in GenericDictionary

Unity 6000.6 promotes its script-serialization rules to compiler diagnostics. GenericDictionary's two private lookup fields are marked [SerializeField, HideInInspector], which now fails the build outright:

Runtime/Utility/GenericDictionary.cs(22,39): error UAC1016: Field 'indexByKey' uses dictionary type
  'System.Collections.Generic.Dictionary<TKey, int>' whose key type 'TKey' is not serializable.
Runtime/Utility/GenericDictionary.cs(24,42): error UAC1016: Field 'dict' ... key type 'TKey' ...
Runtime/Utility/GenericDictionary.cs(24,42): error UAC1016: Field 'dict' ... value type 'TValue' ...

Fix: mark them [NonSerialized].

-        [SerializeField, HideInInspector]
+        [NonSerialized]
         private Dictionary<TKey, int> indexByKey = new Dictionary<TKey, int>();
-        [SerializeField, HideInInspector]
+        [NonSerialized]
         private Dictionary<TKey, TValue> dict = new Dictionary<TKey, TValue>();

Those attributes were never doing anything. Unity has no Dictionary<,> serialization support on any version, so no YAML was ever emitted for these two fields — only list is serialized, and OnAfterDeserialize rebuilds both lookups from it. The change just states that existing intent explicitly.

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Compatibility

Neither fix changes behavior on the Unity versions Scopa already supports (package min 2022.3):

• Fix 1 is behind #if UNITY_6000_5_OR_NEWER; older editors compile the original line.
• Fix 2 needs no guard. Since the attributes were inert, serialized layout is unchanged on every Unity version — existing .asset / .prefab / .unity files keep round-tripping identically, and no reimport or migration is required.

Testing

Headless batchmode compiles of throwaway projects consuming this branch as a local package:

Editor	Result
6000.5.4f1	exits 0, zero compile errors
6000.6.0f1	exits 0, zero compile errors

On 6000.6.0f1 I also ran the negative control: reverting only the GenericDictionary commit reproduces exactly the three UAC1016 errors above and fails the build, confirming that change is what clears them.

The 6000.6 build additionally emits two pre-existing UAC1002 warnings (ScopaEntityData's Sledge base classes Entity / MapObject lack [Serializable]). Those are warnings, not errors, they predate this PR, and fixing them means editing the vendored Sledge.Formats source — so I've deliberately left them out of scope here. Happy to open a separate PR if you'd like them addressed.